On-Site vs. Off-Site Work

Question: Do you charge differently for onsite time versus offsite work? I have been considering trying a strategy like that to get past this hang-up that some clients seem to have with regard to my being onsite.
Here are some responses to this question:
I had a trainer working for me once who used a "pantyhose surcharge" on her per diem whenever she was required to do course development work at the client site (she worked in a track suit at home). It didn't seem to change the client's requirements but made the extra effort and time a bit more remunerative.
I love telling the story about working for Lockheed Martin. My first assignment was onsite by necessity. For a second assignment, they called and had a less generous budget and wanted to know how I could help them and yet reduce the cost. I replied, "Well, if we do it your way, it will cost $6500, and if we do it my way, it will cost $3000." "Gee, what's the difference?" "Well, your way, I get on a plane to Syracuse, work onsite, and then go home. My way is you send me the files, I work on them, we set up a phone conference to go over them, and then I make any changes and send them back to you." "You mean you charge extra for being here in person?!?" "No, that's just the airfare and hotel from my last trip to see you." "Well, in that case, we'll do it your way!"
Next thing I know, a consultant calls me to complain that I've ruined Lockheed Martin. They are now asking everyone for their "site" and "virtual" pricing. He saw no advantage in staying out of airports and hotels – I guess his wife didn't like having him around the house.